1. Field of the Invention
The present invention relates to a piezoelectric resonator coupled to an oscillating amplifier (“crystal oscillator”) and, more specifically, to a crystal oscillator embodied on the same monolithic substrate as a frequency synthesizer to offset any frequency skewing caused by, for example, temperature or fabrication process fluctuations.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
Within nearly every electronic subsystem is some sort of generator that produces cyclical waveforms. The waveform generator is oftentimes referred to as an oscillator that produces a regular oscillation voltage or current. Depending on the application, an oscillator can be used to source regularly-spaced pulses or clock signals. Oscillators are generally rated depending on their stability and accuracy, frequency adjustability (i.e., tunability), and power consumption.
There are numerous types of oscillators in the marketplace. A simple kind of oscillator is an RC relaxation oscillator. A more stable and accurate oscillator, however, involves the use of a piezoelectric resonator, oftentimes referred to simply as a piezoelectric crystal or “crystal.” A crystal implements the piezoelectric effect of converting mechanical vibrations into electrical impulses and vice-versa. If alternating currents or voltages are applied to the crystal, it will vibrate at a resonant frequency, and harmonic modes thereof. To initiate and amplify the piezoelectric effect, a circuit is coupled to the crystal and generally consists of an amplifier with feedback. The frequency of feedback is governed by the low impedance, resonant frequency of the crystal, and the resonant frequency (with harmonics) is governed by the type of crystal used, the orientation angle at which the crystal is formed or cut, and the transducer used to convert acoustic waves in the crystal to electrical impulses and vice-versa.
While there are many types of piezoelectric resonators, there are also many types of oscillating amplifiers coupled to the resonator. For example, the oscillator can be voltage-controlled. Crystal-based oscillators are oftentimes referred to as a voltage-controlled crystal oscillators or VCXOs. A voltage-controlled crystal oscillator can be thought of as an oscillator that can “pull” the frequency of the resonator to a different frequency from that of the resonating frequency. One mechanism used to pull the frequency is to impart an external voltage onto a varactor. The varactor can be coupled to one or both terminals of the resonator to adjust the capacitive value on the inputs. Alternatively, the varactor can be arranged as a bank of selectively coupled capacitors, whose capacitive value is adjusted by, for example, a digital signal.
In addition to voltage-controlled crystal oscillators, many crystal oscillators (XO) can also be temperature compensated. It is generally well known that piezoelectric resonators shift slightly in frequency as the operating temperature changes. In order to compensate for the frequency shift, temperature readings must be taken from the resonator, and the output from the voltage-controlled crystal oscillator must be shifted to offset the temperature skew. Thus, the frequency output from the voltage-controlled crystal oscillator can be pulled to another frequency to minimize, if not eliminate, frequency skewing caused by temperature changes.
To take into account temperature fluctuations, many conventional solutions employ a crystal oven. By placing the crystal into an oven separate and apart from the amplifier/oscillator circuit, the resonator is maintained at a constant temperature with little, if any, resonant frequency fluctuations. Alternatively, a temperature sensor can be coupled to the resonator and electrical leads extending from the resonator case to the amplifier/oscillator circuit that is spaced from the resonator. In each solution, however, the piezoelectric resonator must be housed in a specifically designed package outside of and external to an integrated circuit on which the oscillator/amplifier is formed. Electrical leads from the temperature sensor thereby extend along relatively long capacitive- and inductive-loaded leads to a separately packaged integrated circuit in which the amplifier is located.
As described herein, an integrated circuit is one that is formed on a single, monolithic substrate. The integrated circuit is formed by dicing die from a wafer and then packaging a dice removed from the wafer within a package containing leads extending from the package. Typical voltage-controlled crystal oscillators or temperature-controlled crystal oscillators (TCXO) utilize an integrated circuit on which the amplifying oscillator circuit is arranged in an integrated circuit that is separate from the piezoelectric resonator, where both are coupled by trace conductors extending from a lead of one package to the other. In addition to the loading of the temperature readings through a relatively lengthy set of trace conductor leads, and sensors placed on the external case of the resonator (rather than on the resonator itself), other disadvantages might exist. For example, the amount by which the frequency can be pulled in a typical solution is somewhat limited.
It may be desirable to change the output frequency from the oscillator by either programming the oscillator during its manufacture or possibly in the field with a more variable and flexible frequency outcome. It may also be desirable to implement field and mask programmability not only in a voltage-controlled crystal oscillator, but also for use in a temperature-controlled crystal oscillator where extremely small temperature fluctuations can be accounted for using a high resolution programmable table, modulator, and feedback dual or multi-modulus divider.
In addition to the desirability of having a more tunable output frequency that takes into account temperature fluctuations, it would also be desirable to introduce the temperature sensor directly onto the resonator and integrating the resonator onto, partially within, or within the same substrate which bears the amplifying oscillator circuit. This will shorten leads between the resonator and associated circuitry, as well as lessen the loading effects of the conventionally long leads and the deleterious, somewhat inaccurate readings taken from distally-coupled temperature sensors.